A high-performance, single-focus or zoom lens unit for single-lens reflex cameras, etc. generally has about 10-40 lenses in a lens barrel. Each lens has a laminated anti-reflection coating comprising pluralities of dielectric layers each having a different refractive index from that of a lens substrate and a thickness of ½λ or ¼λ, wherein λ is a center wavelength, to utilize interference effects. The anti-reflection characteristics are important to a wide-angle lens, to which the angle of incident light is large particularly in its peripheral portion.
For instance, when an anti-reflection coating formed on each of 20 lenses has reflectance of 0.5%, its transmittance is 0.99540=0.818 because the number of lens surfaces is 40, resulting in reflection loss of about 18%. Because reflection is superimposed in and between the lenses, large reflection loss of each lens provides the resultant photographs with flare and ghost, as well as reduced contrast. Accordingly, an anti-reflection coating with small reflection loss should be formed on a lens used in single-focus or zoom lens units.
In addition, blue tarnish, white tarnish, etc. may occur on the surface of a lens during a production process. The blue tarnish is a thin film formed by basic components in optical glass dissolved into dew attached to a surface of the optical glass left in the air, or water during a grinding step. The white tarnish is white blot generated by the chemical reaction of components eluted from the glass. Accordingly, the anti-reflection coating formed on a glass lens should have a function to prevent tarnish.
JP 5-85778 A discloses an optical member comprising an anti-reflection coating having pluralities of dielectric layers on an optical substrate, the innermost layer being made of SiOx (1≦x≦2) and having a thickness nd of 0.25λ0 or more, wherein λ0 is a designed wavelength. However, this anti-reflection coating has poor anti-reflection performance, and fails to fully prevent tarnish.
JP 10-20102 A discloses an anti-reflection coating comprising first to seventh layers formed in this order on a substrate having a refractive index of about 1.4-1.9 at a reference wavelength λ0 arbitrarily designed in a visible wavelength range, the first to seventh layers meeting the following conditions: 1.35≦n1≦1.80, 1.90≦n2≦2.50, 1.35≦n3≦1.80, 1.90≦n4≦2.50, 1.35≦n5≦1.80, 1.90≦n6≦2.50, 1.35≦n7≦1.50, 0.0450λ0≦n1d1≦0.2500λ0, 0.0450λ0≦n2d2≦0.1300λ0≦0.0450λ0≦n3d3≦0.1100λ0, 0.2100λ0≦n4d4≦0.3000λ0, 0.0450λ0≦n5d5≦0.0750λ0, 0.1000λ0≦n6d6≦0.1600λ0, 0.2350λ0≦n7d7≦0.2900λ0, wherein n1-n7 are the refractive indices of the first to seventh layers, and n1d1-n7d7 are the optical thickness of the first to seventh layers. Although this anti-reflection coating has a tarnish-preventing function because the first layer is made of alumina, its reflectance to visible light is as large as about 0.3% because the seventh layer is made of MgF2.
JP 2001-100002 A discloses an anti-reflection coating having a 10-layer structure comprising a MgF2 layer, a ZrO2/TiO2 layer, an Al2O3 layer, a SiO2 layer, a ZrO2/TiO2 layer, a SiO2 layer, a ZrO2/TiO2 layer, a SiO2 layer, a ZrO2/TiO2 layer, and an Al2O3 layer in this order from the surface, which has reflectance of about 0.1% at a visible wavelength of 270 nm. JP2002-107506 A discloses an anti-reflection coating having a 10-layer structure comprising a MgF2 layer, a ZrO2/TiO2 layer, a SiO2 layer, an Al2O3 layer, a ZrO2/TiO2 layer, a SiO2 layer, a ZrO2/TiO2 layer, a SiO2 layer, a ZrO2/TiO2 layer, and an Al2O3 layer in this order from the surface, which has reflectance of about 0.1% at a visible wavelength of 300 nm. However, because of the outermost MgF2 layer having as high a refractive index as 1.38, both anti-reflection coatings should have as many as 10 layers for sufficient anti-reflection characteristics.
JP 2005-352303 A discloses an anti-reflection coating comprising pluralities of layers each having a physical thickness of 15-200 nm, which are formed on a substrate such that their refractive indices decrease gradually from the substrate side, the refractive index difference between adjacent layers and between the innermost layer and the substrate being 0.02-0.2, and the outermost layer being a silica aerogel layer. However, it has as large reflectance as more than 0.5% to visible light near a wavelength of 400 nm, and fails to fully prevent tarnish.
JP 2006-3562 A discloses an anti-reflection coating comprising pluralities of layers each having a physical thickness of 15-200 nm, which are formed on a substrate such that their refractive indices decrease gradually from the substrate side, the refractive index difference between adjacent layers and between the innermost layer and the substrate being 0.02-0.2, the innermost layer being an alumina layer, and the outermost layer being a silica aerogel layer. Although this anti-reflection coating has a tarnish-preventing function because the innermost layer is made of alumina, it does not have sufficient reflectance to visible light near a wavelength of 400 nm.
JP 2007-94150 A discloses an anti-reflection coating having 5 or 6 layers, the outermost layer being a silica aerogel layer, which has reflectance of 0.05% or less to visible light in a wavelength range of 400-700 nm at an incident angle of 5°. However, because the innermost layer is made of Ta2O5 or ZrO2, tarnish cannot be fully suppressed unlike alumina.